In the industry of dyeing materials, particularly textiles, the requirements for producing colour matches are becoming more stringent and the work entailed in formulating the requisite dye mixes more labourious and time consuming. To reduce the amount of work required to produce the requisite dye mixes, methods have been evolved intended to predict, from measurement of the target colour, the dye mixture capable of producing this colour.
The usual known method of making such a prediction is, first of all, to make a selection of dyes likely to produce the target colour, then apply each dye to a given substrate with different concentrations covering the range of working concentrations normally used. Usually at least six different concentrations are used.
The reflectance values of the different dyed samples at the different concentrations are measured using a reflectance spectrophotometer. The measurements are made at different wavelengths spread over the normal visible wavelength range. Suitable wavelength intervals are intervals of 20 nanometers over the range of wavelengths 400 to 700 nanometers.
The reflectance values for each dye at each concentration and each wavelength are usually each converted to a function of reflectance, and the relationship of each function of reflectance value against dye concentration is obtained.
An estimate of the reflectance value of an arbitrarily chosen mixture of the single dyes is made from the functions of reflectance relationships of the single dye samples. From these functions of reflectance relationships, and making allowance for the reflectance value of the undyed substrate, the tristimulus values of the colour produced by the arbitrary mixture are obtained, usually in the form XYZ values as defined by the Commission Internationale d'Eclairage (CIE).
These tristimulus values are compared with tristimulus values obtained experimentally from the given colour and if the two sets of tristimulus values are sufficiently close together then the dye formulation as represented by the arbitrary concentrations chosen is considered to provide a match. If the two sets of tristimulus values are not sufficiently close together to give an acceptable match then a new dye mixture having the same single dyes but with different concentrations is formulated and the tristimulus values of the new dye mixture are compared with the tristimulus values of the target colour. This operation is repeated until the match is considered to be satisfactory.
In practice there is usually a non-linear relationship between reflectance values and the concentrations of dyes giving these reflectance values. This is because, as the dye concentration in a mixture is increased, there is usually a tendency for less dye to pass on to the substrate and more to be retained in the dye bath. This non-linear relationship makes it hazardous to perform dye mixture predictions by interpolation of the single dye reflectance against single dye concentration relationships.
The known method described above produces a success rate of dye prediction of normally around 40-60% i.e. about 40-60% of the predictions made produce dye mixtures which are considered to give satisfactory matches to target colours. However, the large percentage of predictions which do not provide a sufficiently close match makes it necessary under present practice to make trial dyeings of each predicted mixture and correct the trial dye mixtures several times if necessary if these are found to provide an unsatisfactory match.
Another serious difficulty arises when attempts are made to predict dye mixes on the basis of measurements made on single dyes. This is because, frequently, one dye will change the dyeing ability of another dye with which it is mixed, and thus no matter how accurately the dyeing behaviour of any single dye is known it is almost certain that this dye when mixed with other dyes will not behave exactly as it does when used alone. The presence of the substrate and/or changes in the characteristics can also affect the behaviour of dyes whether singly or in admixture.
The methods customary in the art for predicting a dye mixture to provide a given target colour take no account of the interactions of single dyes when these are mixed and there is no obvious way of improving their accuracy of prediction.
It would be of great advantage in the dyeing industry to have a method of formulating dye mixtures to produce given colours on given substrates by prediction, using, as the starting point, dye samples actually produced in the factory or produced from dye mixtures in the laboratory under conditions as closely related to factory conditions as possible, if such a method would have a much greater success rate in producing dye mixture predictions.